Tamoxifen-resistant cells display accelerated growth rate and decreased sensitivity to fulvestrant. (A) Cells were grown in complete media in the presence or absence of 10 μM 4-hydroxy tamoxifen (Tam) for 24, 48, 72 and 144 hours, and (C) in estradiol (E2)-free media for 48 and 96 hours and supplemented with vehicle (C) (DMSO), 1 nM E2, 1 μM Tam and 0.1 μM fulvestrant (Ful). Fold increase in cell counts normalized to zero hour counts of respective cell lines are represented. (B) Cells were grown in the presence of DMSO or 10 μM Tam in complete media. BrdU substrate was added 48 hours after drug treatment and assayed after 24 hours. (D) Cells were treated with increasing Ful concentrations for 24 hours and evaluated for estrogen receptor (ER) protein. Presented is a representative western blot. The bar graph represents densitometry analysis of the ER/GAPDH ratio, normalized to the respective untreated controls, from two independent experiments. (E) Tumor volumes of MCF7 and tamoxifen-resistant cells derived from MCF7 (TAMRM) cell mouse xenografts treated over time with vehicle or Tam are represented. (F) T47D and tamoxifen-resistant cells derived from T47D (TAMRT) cells were treated with increasing Tam doses for 72 hours. Proliferation was determined and normalized to respective untreated controls. For graphs, averages are presented with error bars indicating the standard error of the mean. Asterisk (*) indicates significant difference (P value < 0.05), compared to control group. Dt, doubling time.